This invention relates to a method for imparting color to a contact lens. More specifically, it relates to an improved method for preparing a highly pure compound of a hydrophilic monomer and a dye.
The conventional method for imparting an evenly dispersed tint in a soft contact lens is described, for example, in U.S. Pat. No. 4,468,229. Generally, the lens is first soaked in an aqueous solution of the dye, and then the dye is bonded to the lens in a separate solution. The lens is typically composed of a hydrophilic polymer derived from the polymerization of hydrophilic monomers. The bonding of the dye to the lens is carried out by contacting the soaked lens with an aqueous base prior to the final hydration step, which is intended to provide the soft, hydrogel lens with the desired amount of water at an acceptable pH.
The dyes which are used in the conventional method are typically derived from a halotriazine such as a dihalotriazine or monohalotriazine, especially water-soluble dichlorotriazines. Dichlorotriazine or monohalotriazine dyes that carry sulfonate functionalities, for example, are soluble in water, so it is necessary that bonding occur with the hydrophilic polymer from which the lens is composed before the final hydration step. Otherwise, the dye could migrate within the lens to create an uneven dispersion, or leach out from the lens into the eye of the wearer.
The dye which imparts the tint to a soft lens made using the conventional method not only is dispersed in the lens, but also does not migrate within the lens or leach out of the lens after the bond has formed. The tinted lens is stable in an aqueous medium, and after repeated high temperature cycling, conditions which are present during routine wear and cleaning. The conventional method requires that the lens be soaked in a solution containing the dye which is at a specific concentration, and at a specific conductivity, so that the dye diffuses into the polymer. The conductivity is important since one may control the swelling of a lens by selecting various salt concentrations. It is also important that the dye concentration and time the lens stays in the dye soak be precisely controlled since the diffusion kinetics determine the intensity of the tinted contact lens. The conventional method employs a high concentration of dye in the dye wash so that the continuous tinting can be managed. Unfortunately, this method is cumbersome and requires multiple steps, especially at commercial scale production, because it is necessary to soak the lens in a solution of the dye at a specific concentration and time to create a dispersion of the dye in the lens. Therefore, because of this difficulty, alternative methods have been sought.
U.S. Pat. No. 4,559,059 mentions that it might be possible to react a monomer such as 2-hydroxyethyl methacrylate with a reactive dye prior to polymerization, and subsequently to polymerize the monomer dye units during polymerization of the monomers from which the lens is derived. However, this patent does not describe the method nor reaction conditions necessary to prepare the polymerizable monomer-dye units.
U.S. Pat. No. 4,157,892 discloses adding a functionality to the polymer from which the lens is derived which is reactive with the dye. The functionalized polymer is prepared by reacting a "coupler monomer" with a conventional hydrophilic monomer. This coupler monomer has a high probability of changing the physical properties of the polymer. The lens prepared from the functionalized polymer is immersed in a solution of a diazonium dye, where the dye then bonds to the polymer. Although adequate bonding occurs, this method still requires immersion of the finished lens in a solution of the dye.
Another method for imparting color to a soft lens is disclosed in U.S. Pat. No. 4,640,805. This patent describes preparing a tinted lens using a conventional spin casting technique. A suspension of dye pigment in liquid monomer is applied to the mold surface prior to polymerization of bulk monomer in the spin cast mold. Although this method provides a simple way for imparting color to the surface of the lens, it does require that the mold be stamped or printed with specific geometries and spacing.
Attempts have been made to incorporate the dye in the lens by polymerizing the hydrophilic monomer from which the lens is derived in the presence of the dye. For example, U.S. Pat. No. 4,252,421 discloses polymerizing a hydrophilic monomer in the presence of a water-insoluble phthalocyanine dye. The dye is supposed to become entrapped in the finished, hydrated lens because of its incompatibility with water. Unfortunately, the dye will leach out of a lens derived from polymerizing the most commonly used hydrophilic monomer, hydroxyethyl methacrylate (HEMA), when the lens is fully hydrated to greater than about 40 weight percent water. This is even more of a problem with higher water content materials.
The '421 patent also discloses functionalizing the dye with a polymerizable vinyl group, and then subsequently bonding the functionalized dye during polymerization of the monomers from which the lens is derived. Although this eliminates the need for a post-bonding step, the water content of the lens is adversely affected unless hydrophilic --SO.sub.3 H or --SO.sub.3 Na groups are added to the phthalocyanine dye nucleus (as discussed at column 8 of the patent). This simply adds another burdensome step in the manufacturing process to make a contact lens suitable for extended wear applications.
In a similar manner, European Patent Application 0 396 376 discloses the use of a non-charged anthraquinone dye which is functionalized with a polymerizable group to facilitate bonding of the functionalized dye during polymerization of the hydrophilic monomer. Unfortunately, the non-charged dye leads to lower water solubility, if any at all, which in turn restricts the concentration of the dye which can be present in the lens. More importantly, however, the functionalized anthraquinone dye is by necessity a difunctional dye in this case. This difunctionality creates in effect a dye which is a crosslinker. As a result, the water content of the lens is further lowered, and lenses made with this difunctional dye are unacceptably brittle when the concentration of the dye in the lens is increased.
Finally, another attempt to impart color to a contact lens is disclosed in U.S. Pat. No. 4,639,105. This patent discloses spin casting a mixture of liquid monomer, soluble dye and pigment particles to prepare a lens with variations in color achieved by migration of the pigment particles during spin casting. Although this patent indicates that the dyes do not migrate, no reference is made of what specific dyes are used, and it is believed that such dyes will indeed migrate or leach during wear unless the dye used is functionalized with polymerizable groups as described above. Furthermore, such a lens is unsuitable for those applications where a uniform dispersion of dye or colorant is necessary or desired.
U.S. Pat. No. 4,795,794 discloses monomer-dye intermediates by reacting substituted anthraquinones with methacryloyl chloride to produce colored methacrylate compounds (monomer-dye intermediates) which were further polymerized with styrene and other methacrylates to produce polymers used as colored toner particles for copier applications. '794 separates the monomer-dye intermediates by methyl alcohol quench, vacuum concentration, methyl alcohol wash and filtration followed by recrystallization from either methyl cellosolve or methyl alcohol. The recrystallization step is extremely tedious and hazardous.
A drawback with typical reactions for coupling a dye to a hydrophilic monomer unit stems from the impurity of the commercially available dyes used to dye contact lenses. These impurities range from high concentrations of inorganic salts to surfactants used as antidusting agents or to speed dissolution of the dyes. The above components are added, because the major application of these dyes is for the tinting of cellulosic textile materials. These additives make the reactive dye more functional in the textile processes. Synthetic impurities include dye precursors and reaction by-products. The usual result of reacting these dyes with a hydrophilic polymer is a very low percentage of dye attached to the polymer relative to the amount of dye used, and then the unreacted materials and impurities must be washed out of the lens. The washing step is an additional processing step to complicating the production of colored contact lenses. Thus, it is an object of the present invention to provide a highly purified monomer-dye unit. It is also an object of the invention to provide a simple method for making a highly purified monomer-dye unit, which can be used to color contact lenses.
In view of the deficiencies of the prior art, an economical method is needed to prepare a tinted contact lens without requiring the step of immersing the finished lens in a solution of the dye.